Magnetic Susceptibility, Biostratigraphy, and
Sequence Stratigraphy: Insights into Timing of Devonian Carbonate Platform
Development and Basin Infilling, Western Alberta*
By
Michael
T. Whalen 1
and Jed Day 2
Search and Discovery
Article #30033 (2005)
Posted September 1, 2005
*Oral presentation at
AAPG Annual Convention, Calgary, Alberta, June 19-22, 2005.
Click
to view presentation in PDF format.
1University
of Alaska-Fairbanks, Fairbanks, AK ([email protected])
2Illinois
State University, Normal, IL
Abstract
High-resolution magnetic susceptibility (MS) data from
slope and basin deposits near Miette and Ancient Wall provide insight into
patterns of carbonate platform development and infilling of the Devonian Alberta
basin. These MS data, combined with conodont-brachiopod biostratigraphic data
and sequence stratigraphy, provides additional control on the relative timing of
five major and fifteen higher frequency MS excursions and nine depositional
sequences, seven of which correlate to Johnson et al. Devonian Transgressive-Regressive
cycles IIa-2 to IIe. Spikes in the MS record coincide to events associated with
lowstand or initial transgression. The first positive MS excursion (Thornton Cr.
Member, Flume Formation, IIa-2) represents initial mid-Devonian (late Givetian)
onlap at Ancient Wall. The remaining Flume consists of two carbonate ramp
sequences with fluctuating MS corresponding to T-R cycle IIb-1 (norrisi Zone)
and IIb-2. The second significant MS spike (Maligne Formation, Montagne Noir
(MN) Zone 4) is associated with an early Frasnian flooding (IIb-3) that resulted
in isolated prograding rimmed platforms (lower Cairn Formation) and lower MS
values. An MS spike near the base of the Perdrix Formation coincides with the
IIc transgression (MN 5) and upper Cairn backstepping. An MS increase in the
upper Perdrix/lowermost Mount Hawk Formation signals lowstand/initial
transgression of an intra-T-R cycle IIc sea-level event (MN 7?-10), that
resulted in platform aggradation (Peechee Member, Southesk Formation). The
lower-middle Mount Hawk Fm. (MN 11-12) displays increasing MS values related to
clastic influx that continued until swamped by increased highstand carbonate
input (Arcs Member, Southesk, IId-1). The uppermost Mount Hawk and overlying
Simla/Ronde Members (Southesk) record fluctuating MS values (MN 13, IId-2). The
early Famennian Sassenach Formation records the initial triangularis Zone
flooding (T-R IIe) with somewhat higher MS values than seen in latest Frasnian
carbonate ramp deposits of sequence 8.
Figures with
Notes
![](thumbs/01.jpg)
Today I’d like to
present some results from ongoing research on the Devonian of western Canada. My
colleague Jed Day and I are building upon earlier sequence stratigraphic
analysis that I, along with colleagues Gregor Eberli, Frans van Buchem, and
Peter Homewood, conducted during the late 1990’s. We have further documented
additional depositional sequences, enhanced our understanding of the
biostratigrpahy that is crucial for correlation regionally and globally and are
now collecting data on magnetic susceptibility of slope and basin sediments to
further improve correlations.
![](thumbs/04.jpg)
During the Late Devonian North America was at near
equatorial latitudes and a system of attached and isolated carbonate platforms
developed in western Canada. Our research has centered on the SE margins of two
isolated platforms in western Alberta. Miette was approximately 165 km2
while Ancient Wall was substantially larger at about 1200 km2. Slope
and basin successions are approximately 400 m thick. The red lines indicate the
location of stratigraphic cross sections that I’ll illustrate and the red dots
indicate the locations of slope and basin successions sampled for magnetic
susceptibility.
![](thumbs/06.jpg)
The stratigraphic nomenclature for the Devonian
differs both between this surface and subsurface and platform and basin. Outcrop
platform units include The Flume, Cairn, and Southesk Formations. The Southesk
is subidivided into the Peechee, Arcs, and Ronde/Simla members. Basinal facies
include the Maligne, Perdrix, Mount Hawk, and Sassenach formations. The Flume
Formation unconformably overlies Cambrian rocks that were transgressed during
the late Givetian.
![](thumbs/18.jpg)
Sequence 1 coincides with TR IIa-2 in upper
disparilis zone. Base of sequence 2 coincides with IIb-1 and the norrisi
zone, but top sequence 2/base sequence 3 cannot as yet be pinned down
biostratigraphically but could potentially coincide with IIb-2. If so, Sequence
4 represents another intra-IIb sea level event (IIb-3). Sequences 5 and 6
correlate biostratigraphically with TR cycle IIc (MN zones 5-10). The boundary
between 5/6 cannot be pinned down accurately but appears to fall in zone 7. The
top of sequence 6 coincides with the base of IId-1 at the base of MN 11.
Sequence 7 correlates with IId-1 and spans MN 11-12. Sequence 8 correlates with
MN IId-2 and was deposited during MN 13. The lowermost portion of the middle
triangularis zone is unrepresented, but sequence 9 correlates with the lower
portion of IIe.
![](thumbs/19.jpg)
MS measurements
courtesy Brooks Ellwood, LSU on sensitive Kappa bridge. MS signal is controlled
by detrital magnetic and paramagnetic mineral content. Most of the detrital
material in these rocks is paramagnetic and susceptibilities are very low. It
has been postulated that the MS signal in deep marine rocks is generally
controlled by global sea level and has been proposed as an oxygen isotope proxy
in Tertiary deep marine sediments.
![](thumbs/20.jpg)
Mineralogy of the Devonian basinal section at Marmot
Cirque and MS data from Poachers Creek. Sections are directly correlative but
exposed in two different thrust sheets along the SE margin of the Miette
Platform. Total % carbonate and detrital sediment determined with a carbonate
bombe. TOC by LECO carbon analyzer. Note that the major MS excursions are
directly related to decreased carbonate and increased detrital input.
![](thumbs/22.jpg)
We have documented five long term and 16 short term
MS events. Events defined by identifying either maximum or minimum MS values in
any one portion of the curve.
![](thumbs/23.jpg)
The Thornton Creek Member represents a new member of
the Flume Formation identified along the SE margin of Ancient Wall. It
represents the earliest marine transgression of the west Alberta Ridge in
western Alberta. It records three carbonate-siliciclastic parasequences, each of
which is capped by a more siliciclastic rich facies.
![](thumbs/25.jpg)
Base of section exposes sequence 1 with slightly
elevated MS signal. Other major MS events are associated with lowstand or early
transgressive intervals.
![](thumbs/27.jpg)
All five long-term MS events can be correlated
between the area east of Miette to Ancient Wall. Due to cover, not all 16
short-term MS events can be recognized in all sections. Dashed lines are
sequence boundaries.
![](thumbs/28.jpg)
Spline smoothed MS signal from Late Devonian condensed
sections in Morroco (from Ellwood et al., 1999) compared to data from western
Alberta. Morrocan sections are very thin condensed intervals that were measured
at 5 cm intervals. Outcrop data, from much thicker sections, were sampled at 0.5
m intervals. Despite the difference in lithofacies and extreme distance, major
MS events constrained by biostratigraphy can be correlated.
![](thumbs/29.jpg)
Recent O isotopic data collected from brachiopods in
Iowa, Missouri, and Manitoba provide data for comparison with our MS trends. Two
small positive excursions in the Upper disparilis zone and MN zone 2
appear to coincide with Sequences 1 and 2 in the Flume Formation. A broad minor
positive excursion from MN 9 through 11 corresponds to the major MS increase in
the uppermost Perdrix and Mount Hawk (Sequences 6 and 7). A small positive
18O excursion in zones 12-13 appears to coincide with upper Sequence 7 and
8. Positive excursion near the F-F boundary coincides with MS increase in
uppermost Ronde/Simla and lower Sassenach.
Conclusions
-
Nine depositional sequenced deposited from late
Givetian to early Famennian.
-
Five major MS excursions, 16 higher frequency
events.
-
MS signal – detrital proxy, highest during late
HST, LST, or early TST.
-
Provides higher resolution subdivision than biostratigraphy but is dependent
on biostratigraphy for temporal control.
-
MS
signal useful for long range correlation and may be a proxy for O isotopes
or eolian input to the deep sea.